Depends on what platform you are running on, due to the software that comes bundled with some interfaces.
If you are using a PC, you can get an interface with no bundled software and still use something like Reaper, which is pretty much all you need to get started.
On the Mac, however, Audacity is your best bet, and doesn't have capabilities like realtime plugin processing. The Presonus Firepod is 400, but comes with Cubase LE and a number of addons. Just more to think of.